Cracking severity is an important parameter for use in determining the optimum operating conditions for cracking furnaces. In a cracking system, a feed mixture is fed to a cracking furnace, which contains at least one cracking tube. The cracking tube is heated to high temperatures, so that the feed mixture decomposes into various constituents. The constituents together are known as crack gas or effluent. Cracking severity, which is the extent to which the feed mixture is broken down, is controlled by altering the temperature of the cracking furnace, altering the residence time of the feed and constituents inside the cracking tubes, or changing the steam dilution in the case of steam cracking, in order to optimize the composition of the effluent.
The tubes in a cracking furnace are typically very hot, around 1000 degrees centigrade and higher, depending on the location within the furnace. Usually, the crack gas is cooled after exiting the furnace. However, the crack gas is still hot after cooling, at around 350 degrees centigrade and higher. Deposition of materials such as carbon usually occurs on the tubes and process lines at such high temperatures, a problem known as xe2x80x9ccokingxe2x80x9d, or xe2x80x9ccoke depositionxe2x80x9d. Coking has interfered with the ability to accurately measure the concentration of the constituents within the tubes and process lines, because the sample lines or sampling systems for instruments used online often become dirty and plug as a result of coking.
The coking problem has greatly hampered the reliable analysis of the cracking gas, thus preventing improved process control based on cracking severity. Cracking furnace operators have tried to develop different methods in order to control cracking severity. Often, a portion of the effluent of the cracking process is brought outside of the cracking process lines in order to analyze the composition of the effluent. Such a procedure sometimes requires heating of the sampled effluent line to prevent condensation of the heavies or tars before the transport of the non-condensed part of the effluent to the analyzer, because the heavies can cause plugging. Another procedure might consist of condensing the effluent and removing the heavies before the effluent is transported to the analyzer. These procedures generally employ gas chromatograph, mass spectroscopy, or a combination of gas chromatograph and mass spectroscopy to analyze the effluent. However, the resulting measurement can be unreliable, because a troublesome sampling system and removal of heavies can result in plugging problems.
Another procedure commonly used for determining effluent composition is to predict the effluent composition from the composition of the feed stream. This determination typically begins with an off-line analysis of the feed stream composition in the laboratory using near infrared (NIR) analysis. The NIR analysis is then used to predict the composition of the effluent based on a mathematical model. See, for example, U.S. Pat. No. 5,452,232, U.S. Pat. No. 5,082,985, and U.S. Pat. No. 5,475,612. However, the accuracy of the prediction of the effluent composition using this procedure depends on the model used. Numerous parameters, such as the type and condition of the furnace, temperature, pressure, steam dilution, and coke deposition, must be taken into account in order for the model to be sufficiently precise. Furthermore, if the composition of the feed stream changes, a new model must usually be developed. Therefore, such mathematical models are often inaccurate and insufficiently reliable for controlling cracking severity.
The above-described procedures for measuring the composition of the feed stream on the inlet of the cracking furnace typically employ spectrometric methods, such as NIR, to analyze the particular stream of interest. The advantages of spectrometric methods are that no sample handling is required, and such measurements can occur essentially instantaneously. However, spectroscopic methods have not been performed directly on the outlet of the cracking furnace tubes, due to the above described coking problems.
It would be an advance in the art of cracking severity control if a method could be developed for analyzing furnace effluent directly in the outlet of the cracking furnace tubes, thereby eliminating sample handling concerns and avoiding the need for a mathematical model. The resulting measurement would be direct, reliable, precise, and nearly instantaneous.
The instant invention solves the above mentioned problem of unreliable cracking severity control to a large degree. The instant invention provides for direct online analysis of cracking furnace effluent, without any sampling requirements.
In one aspect, the instant invention is a method for controlling cracking severity in a cracking furnace, the cracking furnace having at least one cracking tube, the cracking tube being heated to a temperature, the cracking tube containing a feed mixture that is fed into the cracking tube and an effluent that flows from the cracking tube, the feed mixture having a residence time in the cracking tube, the method comprising two steps. The first step consists of determining the near infrared spectrum of the effluent in-line. The second step consists of changing a process variable selected from the group consisting of the temperature of the cracking tube and the residence time of the feed mixture in the cracking tube according to the determination of the first step. The feed mixture can also contain a percentage of steam and when it does, then the process variable can be selected from the group consisting of the temperature of the cracking tube, the residence time of the feed mixture in the cracking tube and the percentage of steam in the feed mixture. Preferably, the near infrared spectrum is obtained by shining a beam of near infrared light first through a protective gas-stream, then through the effluent and then through a second protective gas stream to a near infrared spectrometer. A chemometric treatment is preferably made of the determination of the first step to better control the cracking severity.
In a second aspect, the instant invention is an apparatus for analyzing an effluent in-line, comprising a light source mounted on a conduit for the effluent, a light detector mounted on the opposite side of the conduit from the light source to receive light emitted from the light source, means for sheltering the light source from the effluent, means for sheltering the light detector from the effluent, means for flowing a fluid past the Right source at a higher pressure than the pressure of the effluent; and means for flowing a fluid past the light detector at a higher pressure than the pressure of the effluent.